During their growth, fresh-water sponges release into the medium a dialysable inhibitor (gemmulostasin) which reversibly blocks development of the gemmules at an early phase, prior to the first mitoses.

The incorporation of [3H]labelled precursors into DNA, RNA and proteins has been measured throughout development, in the presence or absence of inhibitor. Autoradiographic controls of the nuclear incorporation of [3H]thymidine have been made. Gemmulostasin erases the peak of [3H]thymidine incorporation that otherwise occurs just before the first mitoses. Its overall effect is to bring about, either directly or indirectly, a high peak in the incorporation of [3H]uridine and [3H]leucine at a time when the incorporation of these precursors is low in the controls.

It is suggested that a causal relation exists between these phenomena.

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